In color grayscale xerographic printing the laser pulsewidth is modulated to produce varying voltages on the drum surfaces and varying toner optical densities. This modulation results in pulsewidth differences on the order of nano-seconds, and corresponding small changes in incident energy to the drum. As the pulsewidth becomes smaller, errors in the absolute pulsewidth become a larger percentage of the total energy incident on the drum and the errors result in a larger visible optical density (OD) variation. This error in the incident energy is a result of normal variation in the manufacturing process of exposure system components and the operating environment, including supply voltage variation. This error results in significant instability in the exposure energy for the small pulsewidths and is very visible in the grayscale highlight regions.
To minimize the variation in component performance, higher quality, more tightly specified, components are necessary. These higher quality, more specified components increase the cost of the system.
It is common for grayscale printer manufacturers to use a lookup table process to compensate for average optical density performance and to generate proper gray scale colors. The average optical density performance is normally based on a sample of typical printers and includes the printer performance at small pulsewidths. Based on the average performance the lookup tables are modified to produce an acceptable average optical density over the grayscale range for the process colors. The process colors are yellow, magenta, cyan, black, red, green, and blue. This type of solution can compensate for average variation between the ideal and actual pulsewidths, but cannot compensate for variation in pulsewidth caused by component variation in individual printers. Typically lookup tables are implemented in the firmware of a printer. The lookup tables are not regenerated to account for variations in components among printers due to manufacturing processes, aging, or temperature change.
Several systems compensate for variation in the grayscale pulsewidths by forcing the user to perform a manual adjustment procedure to minimize the effects of pulsewidth error. This manual adjustment typically involves printing a pattern which compares a dithered or line screened highlight with a pure grayscale highlight. The user then must perform an adjustment to match the two highlights. This method is a usability problem, especially for network printers which are designed for little if any user interaction.
Screens and dithers are used to produce larger areas of color to minimize the apparent highlight density variation lines. This solution is very successful for printing methods which have very good addressability and resolution, but increasing addressability and resolution is very expensive for grayscale printers. With inexpensive color grayscale printers, the use of screens and dithers often produces a noticeable reduction in image quality.
To compensate for variations in laser characteristics an automatic power control (APC) process is carried out. In the APC process the laser diode is turned on before the page is printed and the laser power is measured using the detector in the laser's emitter detector pair. Alternatively an external sensor measures the laser power. This power variation is used to modify the drive current of the laser. This method compensates for the DC variation of the laser power, but not the system variations which affect the small grayscale pulses.
Accordingly, given the forgoing backgrounds relating to pulsewidth errors in laser printers, objects of the present invention are to provide a laser power feedback system for compensation of pulsewidth modulation in a laser printing device.